What is an arbitrary element in Python?

ar·bi·trar·y ˈärbəˌtrerē/ adjective, based on random choice or personal whim, rather than any reason or system.

In the context of your question, "arbitrary element" simply means an element not chosen by you. From the program's perspective, the element was chosen randomly and unpredictably.

Consider:

x = set([1,2]).pop()

x might have the value 1 or 2, but you cannot predict beforehand which one it will be.

Sets are unordered.

The remove command takes the element that you specify.

The pop takes any element. There's no way of predicting which